Annual Cathodic Protection Surveys of Plant Piping and a Diesel Tank at a Power Station, El Paso (2022-2026)

A cathodic protection system is only as good as its last survey. After we designed and commissioned the deep-well system at a power station in El Paso, the owner kept us on for the annual surveys; this page draws on the reports for 2022, issued in early 2023, and for 2026.

The system being checked

Two 40 V, 40 A rectifiers drive mixed metal oxide anodes hung in carbon backfill in two deep wells. They protect the coated fuel gas piping, 6 to 12 inches in diameter, and the coated compressed air and drain piping. The galvanic anodes from the original system are still connected but nearly spent. Each survey starts with the rectifiers and the output of every anode, then a pipe-to-soil survey with GPS-synchronized interrupters cycling the current 4 seconds on and 1 second off, so the instant off readings can be compared with the baseline we took before the system was built.

Groundbeds drying out

At the January 2023 survey the east rectifier put out 15.4 A through a circuit of 2.9 ohms, and one anode near the top of its well carried 10.5 A of that total. By May 2026 the same rectifier, at about the same voltage, delivered only 2.4 A through 19.2 ohms, and the west unit had gone from 9.5 to 11.3 ohms. In the desert soil the wells are drying out. Until the piping is fully isolated, the reports recommend soaking each well through its 1-inch PVC vent pipe for at least two hours every month, and suggest an irrigation system to make that routine.

Why the readings do not tell the whole story

The gas piping is still electrically common with the plant grounding, so the potentials we measure are a weighted average of steel and copper, worst near buildings and skids. Over the years the owner repaired three insulating flange kits on the air piping and excavated anomalies we had marked, one of which was a bare ground wire touching the gas pipe. Shorts remain. At the coalescing filter skids, the metal sheathing over the pipe insulation touches clamps, instruments and conduit, and the gas compressor skids have no insulating flanges between buried and aboveground piping. Low readings near the supplier station still point to stray current from the supplier CP system.

The diesel tank

An at-grade diesel tank has its own impressed current system: an anode mesh over an HDPE liner, with four pairs of permanent reference electrodes, copper sulfate and zinc, under the floor. In the 2022 survey we raised its output from 5.4 A to 7.5 A. In 2026, at the as-found setting, every reading met the 100 mV polarization criterion.

Recommendations that carry over

Cut small air gaps in the insulation sheathing at each skid, add insulating flanges at the compressor risers during the next plant-wide shutdown, inspect the rectifiers at least every two months, and repeat the over-the-line coating survey of the gas piping at intervals of no more than seven years.

Related work

The design and start-up of this system are described in our impressed current cathodic protection project.